Logical Reasoning

PT16 · S2 · Q16 Researchers in South Australia estimate changes

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Researchers in South Australia estimate changes in shark populations inhabiting local waters by monitoring what is termed the "catch per unit effort" (CPUE).

Conclusion (therefore)

The population of shark species X in the waters around South Australia must be at approximately its 1973 level.

Evidence

Since the 1973, the number of shark X's that commercial shark fishing boats catch per hour per km of net has remained fairly constant.

Evaluate

The argument presents a statistic and then concludes an interpretation of it, so we could use our Explain Curious Fact framework.

Curious Fact .... how come commercial shark fishing boats catch the same number of shark X's per hour / per km of net nowadays as they did back in 1973?

Author's Explanation ... the waters in that area still have a similar quantity of shark X's in them.

CPUE is essentially measuring how many sharks we catch each hour, while controlling for how much net we drop into the water. If we dropped a standard net in the water back in 1973, we'd catch about 4 sharks per hour. If we drop a standard net in the water nowadays, we'd also catch about 4 sharks per hour (just making up that number as a concrete example).

We can imagine how if the number were dropping (we used to get 4 per hour, but now we're only catching 2), we might jump to the conclusion that there aren't as many sharks in the water. But it's possible to think of Alternate Explanations ... maybe there are just as many sharks, but they've learned to avoid our net. Maybe we've switched to a less enticing shark bait.

In this case, we're seeing that we are still getting the same number of sharks per hour, so this author is jumping to the conclusion that there are a comparable number of sharks in the water.

Let's try to argue the Anti-Conclusion (which will double as thinking of Alternate Explanations).

Given that we're catching the same number of sharks per hour, how could it be that the population level has changed?

If there are more sharks now than before, then how come we're still only catching 4 per hour? (maybe the sharks have gotten smarter at avoiding our nets / maybe we use a less-enticing bait / we would need a way to say that we are worse at getting them in our net than we used to be)

If there are fewer sharks now than before, then how come we're still able to catch 4 per hour? (maybe we've gotten smarter at finding the sharks / maybe we're using a more-enticing bait / we would need some way to say that we're better at getting them in our net than we used to be)

Goal

Let's look for an answer that helps us argue that there are fewer sharks than before or more sharks than before. Fewer sharks + Better shark-hunting methods could result in a similar catch per hour. Similarly, More sharks + Worse shark-hunting methods could result in a similar catch rate per hour.

16.

Which one of the following, if true, most seriously weakens the argument?

The waters around South Australia
No Impact
It wouldn't change anything whether these sharks are found elsewhere as well or only around South Australia.
2% picked this
The sharks that are the
Unclear Impact: most profitable
We have no idea if this particular species of shark the argument is discussing is among those sharks that are "the most profitable" to catch, so we won't know whether this answer has any bearing on this argument.
6% picked this
A significant threat to shark
No Distinction / No Impact
This is a "timeless" idea. It's saying there is always some level of threat to shark populations that they'll get caught in a net intended to catch something else. But without knowing whether that threat has increased, decreased, or stayed the same since 1973, this answer has no impact on the conversation.
19% picked this
Most of the quotas designed
No Impact
The existence of quotas, i.e. "You may only catch up to 20 sharks per day" has nothing to do with CPUE. CPUE is the idea that boats are currently able to catch 4 sharks per hour. So if there's a quota of 20 sharks, then they'd only be able to do 5 hours of catching in a day before they had hit their quota. But we don't care at all about how many hours they're allowed to be out there catching sharks. We just care about how efficiently they're catching sharks when they're out there (i.e. how many sharks per hour). This is saying quotas are usually based on the weight of the sharks, rather than the number of sharks, but again, we don't care about quotas because we don't care about how long they're fishing before they've hit their quota. We care about how many sharks they can catch in an hour.
24% picked this
Correct
Since 1980 commercial shark-fishing boats
Why this is right
This allows us to argue that the population of this shark species is not at its 1973 level. We can argue that there are actually not as many sharks in the water as before. After all, our methods of finding sharks have improved since 1973, so we should be able to catch more sharks per hour than we could back then. So if our CPUE is still only 4 sharks per hour, despite our better shark-finding techniques, it suggests that there are fewer sharks to be found.
Skill tested: Weaken · how this choice captures the argument's function is the move to repeat next time.
48% picked this

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